1. Field of the Invention
The present invention relates to a shale and salt formation stabilizing drilling fluid comprising an aqueous fluid containing dissolved salts.
2. Prior Art
Drilling of salt and shale formations are very old problems. Numerous methods have been developed which facilitate such drilling, however, many of the methods employ expensive additives or fluids which are environmentally undesirable. The use of saturated salt solutions to prevent enlargement of a borehole by preventing dissolution of the drilled salt is known.
Water-sensitive shales contain various amounts of clay minerals. The clay mineral components may be highly swelling montmorillonite or less water-sensitive kaolinite or chlorite. Generally, shales appear to swell to some extent when contacted by fresh water. The uptake of water by shales has two effects: a volume change as a result of swelling and a strength reduction as the water content increases. This may result in flow of the shale into the well bore or spalling of hard shale, both of which restrict the well bore.
The most successful method of controlling water-sensitive shales uses oil-base drilling fluids. High initial cost and environmental restrictions prevent the use of oil-base drilling fluids in many areas in which shale may be encountered. In this event, water-base drilling fluids are treated to minimize the destabilizing effect of the drilling fluid.
In U.S. Pat. No. 3,017,351, a method for drilling through shale is disclosed. In accordance with that method, water-soluble ammonium and potassium salts are admixed with a water-soluble amine and an acrylamide polymer hydrolyte to provide a drilling fluid that helps prevent clay swelling and softening and also prevent breaking of the shale by mechanical action. The use of such combinations in a clay-based drilling fluid, however, is not practiced because the inorganic salts present would flocculate the clays being used for viscosity and fluid-loss control as disclosed in U.S. Pat. No. 4,142,595.
It is not uncommon to encounter successive salt and shale formations while drilling a well. Conventional practice is to use a drilling fluid which is highly inhibitive to the clay in the shale and then rapidly drill through the salt formation or break over the drilling fluid to a saturated salt system.
It would be desirable to provide a method by which salt and shale formations can be stabilized during drilling with a clay-containing drilling fluid.